Some kinds of semiconductor devices have different kinds of Metal Insulator Semiconductor (MIS) transistors with different driving voltages. In general, transistors with different driving voltages have gate insulating films with different thicknesses, for reasons including the difference in gate withstands a voltage requirement between and among the transistors. A known method for preparing gate insulating films of different thicknesses is as follows.
A first silicon dioxide film is formed by thermal oxidation or some other method on first and second transistor formation areas. The first transistor formation area is an area on which a thinner gate insulating film is formed, whereas the second one is an area on which a thicker gate insulating film is formed.
Then, the portion of the first silicon dioxide film lying on the first transistor formation area is selectively removed.
Then, a second silicon dioxide film is formed by thermal oxidation or some other method on the first transistor formation area, while the portion of the first silicon dioxide film that corresponds to the second transistor formation area is further oxidized; a third silicon dioxide film is formed as a film thicker than the first silicon dioxide film.
As a result, the first transistor formation area is covered with the second silicon dioxide film that serves as a first gate insulating film, and the second transistor formation area is covered with the third silicon dioxide film that serves as a second gate insulating film.
A semiconductor device having three or more kinds of gate insulating films of different thicknesses may also be made, by repeating the procedure described above.